1. Field of Invention
This invention relates to medical devices which are implanted in the human body, particular to a valve which is easy to manufacture, performs reliably, is easy to surgically implant in the human body, and will remain functional for the life of the patient in which it is implanted. The valve of this invention has a unique shape that reduces the possibility that the body will reject the valve when implanted in an eye of a glaucoma patient.
2. Background Discussion
Medical valves are used for many different types of applications. One such application is to treat glaucoma by allowing aqueous humor to flow from the intraoccular chamber of the eye to relieve excess pressure. Thomas C. White in U.S. Pat. No. 4,554,918 has suggested one type of glaucoma valve where the aqueous humor flows from the intraoccular chamber through a tube into an external reservoir. The end of the tube in communication with the reservoir has a small opening in its end. The small opening provides a great deal of resistance to flow of the aqueous humor which is highly viscous. The White valve provides for flow in only one direction, namely, from the intraoccular chamber of the eye to the external reservoir. Upon being filled, the reservoir is pressed by the patient to force the aqueous humor contained in the reservoir through another tube into the body of the patient where it is absorbed.
Another device used to treat glaucoma is discussed by Anthony C. B. Molteno in U.S. Pat. No. 4,457,757. This device includes a plate having a tube that extends into the intraoccular chamber. The aqueous humor from the intraoccular chamber flows onto the surface of the plate and is absorbed by the body. The Molteno plate does not have any pressure controlling mechanism, and it is only a device for releasing intraoccular pressure.
Both of these devices have been used to treat glaucoma, but the White valve suffers from the disadvantage that the patient must manually press the reservoir in order to force the aqueous humor collected in the reservoir to escape and be absorbed by the body. Moreover, although the White valve is designed to open when the intraoccular pressure becomes excessive, the valve's structure is not reliable, because it depends upon a tiny opening in the end of the tube which is very small in diameter and can easily be clogged by particulates. Nor is the White valve very sensitive, because it does not respond to slight changes in pressure to open and close. The Molteno plate overcomes the objections of the manually actuated reservoir, however, it does not employ a valve and could lead to hypotony, that is, the loss of aqueous humor within the intraoccular chamber of the eye.
Glaucoma Valve
The valve disclosed in the above-identified U.S. Pat. No. 5,071,408 (herein referred to as the Glaucoma Valve) has many features that distinguish it from White and Molteno.
The Glaucoma Valve uses a membrane under tension to form a chamber having an elongated, slit-like opening therein. The membrane responds to slight changes in pressure to expand or contract to open or close the opening. When opened, it provides a wide open mouth with parted lips that allows for free flow of fluid though it without any substantial resistance to fluid flow. This feature also substantially reduces the likelihood that the opening will be clogged by particulates. Typically the width of the slit-like opening ranges between 2.500 and 3.500 millimeters, preferably between 2.725 and 2.750 millimeters.
The Glaucoma Valve has a chamber preferably with a trapezoidal configuration to provide a narrow side and a wide side. The slit-like opening is essentially coextensive with the narrow side and an inlet tube is connected to the wide side. Fluid flows into the chamber through the inlet tube. Depending on the fluid pressure within the chamber, the slit-like opening will be either opened or closed. The pressure in the chamber must, however, exceed the tension in the membrane in order to expand the membrane means to open the slit-like opening.
The trapezoidal configuration of the chamber renders the valve highly responsive to slight changes in pressure. The fluid as it enters the chamber at the wide side first flows into a space which has a relatively large cross-sectional area compared to the cross-sectional area of the chamber adjacent to the slit-like opening in the narrow side. This is important because it makes the Glaucoma Valve sensitive to slight changes in pressure and allows it to open very briefly to reduce the pressure in the chamber. When the fluid pressure in the chamber just equals the pressure created by the tension in the membrane means, the slit-like opening is closed. As soon as this pressure increases due to additional fluid flowing into the chamber along its wide side, the membrane means expands and the fluid flows from the slit-like opening. The velocity of the fluid flowing from the opening is substantially higher than the velocity of the fluid entering the chamber at its wide side to decrease quickly the pressure in the chamber and close the Glaucoma Valve. The relative high velocity with which the fluid exits the opening aids in flushing the chamber and reduces the possibility of back flow. Since the rate at which aqueous humor is formed in the intraoccular chamber of the eye is very slow, approximately one drop every three hours, slight increases in volume of aqueous humor result in the valve of this invention opening momentarily and then closing. These unique features of the valve of this invention allow pressure in the chamber to be maintained at 10 millimeters (mm) of mercury (Hg), with an increase in pressure of 0.5 mm of Hg opening the valve. As soon as the intraoccular pressure stabilizes at 10.0 mm of Hg, the valve is totally shut off to prevent the further flow of aqueous humor from draining from the intraoccular chamber. Thus, the cornea never loses its dome-like shape and hypotony is avoided.
The Glaucoma Valve uses two plates which hold between them in tension overlying membrane members which form between them the chamber. The slit-like opening is along adjoining, overlapping edges of the membrane members. Preferably, the membrane members are simply two halves of a thin sheet of silastic material which is folded over upon itself. The two plates each include interlocking members that, upon the plates being pressed together, engage to place the membrane members disposed between the plates in tension. By adjusting the size and positions of the interlocking members, the tension may be varied to provide different valve designs which open in response to different pressures. Moreover, once the tension is established for a specific valve design, this Glaucoma Valve is easily reproducible, allowing this specific valve design to be mass produced without any significant variation in its pressure response from one valve to another. The interlocking members for any specific valve design apply essentially equal tension across the entire width of the membrane. This is desirable to insure repeatable performance. Each of the plates include therein identical trapezoidal shaped depressions. The plates are aligned with each other when joined together so that the two trapezoidal depressions are in registration. The trapezoidal configured chamber is formed when fluid flows between the membranes to expand them outwardly, pushing the membranes outwardly against the walls of the depressions.
The Glaucoma Valve is adapted to be attached to the eye to provide a hinge-like, cantilever action. A pair of suturing holes are provided on the main body of the Glaucoma Valve, one hole on each side of the inlet tube. When the Glaucoma Valve is placed on the eye, two stitches are made, one through each hole to secure the valve to the sclera of the eye. This creates a large distribution area adjacent the Glaucoma Valve to be used for absorption of the aqueous humor.